1. Field of the Invention
The present invention relates to a method for producing a multilayer ceramic circuit board, and more particularly a method for producing a circuit board comprising a glass-ceramic having a low dielectric constant and copper conductors having a low electrical resistivity, enabling high-speed signal transmission.
2. Description of the Related Art
A multilayer circuit board consists of layers of electrical insulators, usually ceramic, and metallic conductors. The ceramic must have a low dielectric constant as well as a high electrical resistivity, high bending strength, low thermal expansion coefficient, and high thermal conductivity. The metallic conductor is preferably copper due to its low electrical resistivity and price. During the firing of the multilayer board, the organic binder included in the ceramic must be decomposed without any residual carbon and the copper must not be oxidized.
U.S. Pat. No. 4,234,367 to Herron et al., assigned to IBM, discloses a method of making multilayer glass structures having an internal distribution of copper-based conductors by firing in a controlled ambient atmosphere of H.sub.2 : H.sub.2 O=10.sup.-4 to 10.sup.-6.5, at a temperature below the melting point of copper. In this method, .beta.-spoduemene or cordierite is preferably used as the crystallizable glass. It is, however, difficult to control the ambient atmosphere, due to the extraordinarily small amount of the hydrogen content.
U.S. Pat. No. 4,504,339 to Kamehara et al., assigned to Fujitsu Limited, discloses a method for producing a multilayer glass-ceramic structure having copper-based conductors therein for use as a circuit board. In this method, a multilayer structure comprises layers of a thermally depolymerizable resin and glass-ceramic containing preferably 40 percent to 60 percent by weight of Al.sub.2 O.sub.3 and layers of a copper-based paste. The structure is fired in an inert atmosphere containing water vapor, the partial pressure of which is 0.005 to 0.3 atmosphere, preferably at 550.degree. C. to 650.degree. C. The structure is then sintered in a nitrogen atmosphere containing no water vapor, preferably at about 900.degree. C. However, if the firing temperature in the inert atmosphere containing water vapor is higher than 650.degree. C., the residual carbon is trapped in the closed pores in which H.sub.2 O vapor is present. The carbon then reacts with the H.sub.2 O to form CO.sub.2. This phenomenon results in bloating of the glass-ceramic.
Japanese Unexamined Patent Publication (Kokai) No. 59-11700 to Ogihara et al., assigned to Hitachi, discloses a multilayer ceramic circuit board obtained by firing a mixture of silica and a noncrystallizable glass in an atmosphere containing nitrogen, hydrogen, and water vapor. However, the bending strength of the board is inevitably lower than that of one formed with crystalline ceramics.